In recent years, solid free-form fabrication processes (also known as additive manufacturing processes) have been developed for producing a physical article directly from an electronic representation of the article. The term “solid free-form fabrication process” (“SFFF”) as used herein refers to any process that results in a three-dimensional physical article and includes a step of sequentially forming the shape of the article one layer at a time from an electronic representation of the article. Solid free-form fabrication processes are also known in the art as “layered manufacturing processes.” They are also sometimes referred to in the art as “rapid prototyping processes” when the layer-by-layer building process is used to produce a small number of a particular article. A solid free-form fabrication process may include one or more post-shape forming operations that enhance the physical and/or mechanical properties of the article. Examples of solid free-form fabrication processes include the three-dimensional printing (“3DP”) process and the Selective Laser Sintering (“SLS”) process. An example of the 3DP process may be found in U.S. Pat. No. 6,036,777 to Sachs, issued Mar. 14, 2000. An example of the SLS process may be found in U.S. Pat. No. 5,076,869 to Bourell et al., issued Dec. 31, 1991. Solid free-form fabrication processes in accordance with the present invention can be used to produce articles comprised of metal, polymeric, ceramic, composite materials, and other materials. The development of solid free-form fabrication processes has produced a quantum jump reduction in the time and costs incurred in going from concept to manufactured article by eliminating costly and time-consuming intermediate steps that were traditionally necessary.
Many solid free-form fabrication processes consist of the basic steps of: (1) applying and smoothing out a first layer of a build material, e.g., a powder, to a vertically indexable build stage; (2) scanning the build material layer with the printing mechanism to impart to it the image of the relevant two-dimensional layer of the article being built; (3) lowering the stage to receive another layer of build material; and (4) repeating steps (1) through (3) until the article is completed. The layer-by-layer construction results in the formation of the desired physical article. Subsequent processing is often employed to enhance the physical properties of the constructed physical article.
The term “printing mechanism” as used herein generically refers to the component of the solid free-form fabrication system that (1) physically imparts the image of the relevant two-dimensional layer of the article that is being constructed onto a construction material that is upon the stage upon which the article is being built, and/or (2) deposits a layer of a construction material in the image of such a two-dimensional layer upon the stage or a previous layer. For example, in the 3DP process, the printing mechanism is a print head comprising one or more print jets and associated scanning and control mechanisms that spray droplets of a binder fluid onto a powder layer to form the image of the relevant two-dimensional layer of the physical article. In the SLS process, the printing mechanism is a laser and associated scanning and control mechanisms that scan a laser beam across a powder layer to fuse powder therein together in the form of the image of the relevant two-dimensional layer of the physical article.
A physical article that is to be constructed by solid free-form fabrication is first represented electronically as a three-dimensional model. Typically, the three-dimensional model is stored in the format of a stereolithographic file or .stl file. Files in this format are referred to herein as “STL files.” An STL file typically comprises of a collection of triangles which sketch out the exterior and interior surfaces of the physical article. Features such as surface normals, i.e., a short ray pointing perpendicularly out from a face of the triangle, are associated with the triangles to indicate which surface of the triangle is facing outward from the physical object. The outward facing surface is sometimes referred to as an “exterior” or “front” face and the inward facing surface is sometimes referred to as an “interior” or “back” face.
Conventionally in solid free-form fabrication, an STL file is operated upon by a program that is referred to herein as a “slicing program.” A slicing program slices the model that is in STL file format along one of three mutually orthogonal axes, e.g., the Z-axis of a set of X-Y-Z axes, to create a stack of two-dimensional layers of a specified layer thickness, i.e., slices. Within each slice, the relevant portion of the model is represented by a set of two-dimensional closed polygons.
The slicing program is typically a separate program, e.g., the Magics RP program which is available from the Materialise NV, Leuven, Belgium. However, a slicing program may also be a subset of a larger program that processes the STL file or functionally similar file into instructions for a solid free-form fabrication machine to construct the physical article. In either case, application of the slicing program results in a binary file which comprises a stack of two-dimensional slices wherein each two-dimensional layer is represented by a set of two-dimensional closed polygons. Such binary files are referred to herein as “slice stack files.”
Traditionally, the control software of the solid free-form fabrication machine utilizes a slice stack file to manufacture the physical article layer-by-layer. Typically, the solid free-form fabrication machine control software transforms each model layer represented in the slice stack file into a set of instructions for controlling the printing mechanism in the creation of the corresponding physical layer of the physical article. These instructions tell the printing mechanism where to cause the build material to be (1) bound together, e.g., through the application of energy from a lasing or electron beam device or through the jetting of a binder from a jet print head, and/or (2) deposited. This operation of the printing mechanism is referred to generically herein as “printing” and these instructions are referred to generically herein as the “printing instructions,” irregardless of the type of printing mechanism that is actually employed.
FIG. 1 presents a flowchart representation of a conventional process for creating a physical article by solid free-form fabrication. In exemplar conventional process 10, STL file data 12 for a model of the physical article that is to be built is input into a slicing program 14. Also input into the slicing program 14 is the selected layer spacing value 16 that is to be applied to the entire model. The slicing program 14 uses this input to create a slice stack file 18. Each planar slice is separated from the next slice by the selected layer spacing value 16. Data from the slice stack file 18 is then input into a storage device 20. Subsequently, the data from the slice stack file 18 is output from the storage device 20 into the control software 22 of a solid free-form fabrication machine. The control software 22 processes the slice stack file 18 data to create printing instructions 24 for causing the printing mechanism 26 to print each layer 28 until the completion of the physical article 30.
There are several drawbacks to the conventional method. Among these are the costs occasioned by the need to utilize a slicing program. These costs include the cost of purchasing or developing, implementing, and/or maintaining the slicing program. They also include the costs of the hardware that must be allocated to the operation and the storage of the slicing program and the resulting slice stack files. They further include the computational costs of utilizing the slicing program and then utilizing the slice stack files. Additionally, there is the cost of the time needed to utilize the slicing program to create the slice stack files.
Another drawback is the loss of detail and other information from the original three-dimensional model of the physical article. Each time a data set representing the model is transformed, some detail and information about the model is lost. Slicing programs attempt to represent the models they are operating upon in terms of particular slice planes. Thus, all information from the original model about details that exist between the slice planes is not captured by the slicing program and is therefore lost. This means the original model is not available from the slice stack file for viewing, moving, scaling, or other operations. This also means that the slice stack file can only be used by a solid free-form fabrication machine that is capable of utilizing the particular slice thickness that was selected in creating the slice stack file and which is capable of using the same printing device indexing steps and other parameters. This limits the portability of the slice stack file from one solid free-form fabrication machine to another.
A few years ago, one of the inventors of the present invention, disclosed in U.S. Patent Application Publication US 2010/0168890 A1 methods utilizing ray casting for converting STL files without the use of a slicing program into instructions to the printing mechanism for the layer-by-layer construction of the physical article. Although those methods were superior in many ways to other conventional STL conversion methods, they involved extensive computations and could result in the loss of some resolution.